//观察者列表对象
function ObserverList(){
  this.observerList = [];
}

ObserverList.prototype.Add = function( obj ){
  return this.observerList.push( obj );
};

ObserverList.prototype.Empty = function(){
  this.observerList = [];
};

ObserverList.prototype.Count = function(){
  return this.observerList.length;
};

ObserverList.prototype.Get = function( index ){
  if( index > -1 && index < this.observerList.length ){
    return this.observerList[ index ];
  }
};

ObserverList.prototype.Insert = function( obj, index ){
  var pointer = -1;

  if( index === 0 ){
    this.observerList.unshift( obj );
    pointer = index;
  }else if( index === this.observerList.length ){
    this.observerList.push( obj );
    pointer = index;
  }

  return pointer;
};

ObserverList.prototype.IndexOf = function( obj, startIndex ){
  var i = startIndex, pointer = -1;

  while( i < this.observerList.length ){
    if( this.observerList[i] === obj ){
      pointer = i;
    }
    i++;
  }

  return pointer;
};

ObserverList.prototype.RemoveAt = function( index ){
  if( index === 0 ){
    this.observerList.shift();
  }else if( index === this.observerList.length -1 ){
    this.observerList.pop();
  }
};

// Extend an object with an extension
function extend( extension, obj ){
  for ( var key in extension ){
    obj[key] = extension[key];
  }
}

//被观察者
function Subject(){
  this.observers = new ObserverList();
}

Subject.prototype.AddObserver = function( observer ){
  this.observers.Add( observer );
}; 

Subject.prototype.RemoveObserver = function( observer ){
  this.observers.RemoveAt( this.observers.IndexOf( observer, 0 ) );
}; 

Subject.prototype.Notify = function( context ){
  var observerCount = this.observers.Count();
  for(var i=0; i < observerCount; i++){
    this.observers.Get(i).Update( context );
  }
};

// 观察者
function Observer(){
  this.Update = function(context){
    console.log('通知内容：' + context);
  };
}


var subject = new Subject();

//添加四个观察者
subject.AddObserver(new Observer());
subject.AddObserver(new Observer());
subject.AddObserver(new Observer());
subject.AddObserver(new Observer());

//被观察者发布通知
subject.Notify('我更新了！');